This invention relates to a conveyor system for transporting workpieces to a working position for processing.
In known liquid dispensing systems with conveyors for transporting printed circuit boards for manufacture, two parallel spaced rails each support a conveyor, such as a belt or chain. The conveyors transport circuit boards to a working position where a liquid dispenser can dispense a liquid on top of the circuit board. Liquid may be dispensed on the circuit board for a number of different reasons including to deposit glue, epoxy, or solder; to provide an underfill material around a mounted packaged device so that the underfill material wicks under the device and thereafter serves as a heat sink; or to provide a viscous encapsulating liquid that hardens over a component such as an unpackaged die. For some of these applications it is desirable to have a heating system mounted underneath the circuit board when the board is in the working position. Such a heater typically includes a plurality of quartz heating tubes that extend in parallel, transverse to the direction of travel. The conveyors then transport the circuit boards away from the working position.
In typical dispensing systems, between the rails and underneath the conveyors is a horizontal metal plate that can be raised and lowered. A number of lifters are fixed to the metal plate with magnets and are thereby adjustable in position on the metal plate. When the circuit board is transported to the working position as sensed by a detector, the metal plate is raised so that the lifters contact the circuit board from underneath and raise the circuit board to a position where the liquid dispenser can dispense the liquid on the circuit board. After the dispenser has completed its work, the metal plate is lowered so that the board is brought back down onto the conveyor to be transported away from the system.
Such a lifting mechanism that pushes up on the board from underneath has several disadvantages. Some boards may have components on the underside of the board, and thus the lifters may have to be carefully repositioned for different boards to avoid contacting such components. The rails that hold the conveyors can be brought closer together or spread farther apart for boards of different widths. If the width of the board changes, the lifters may have to be repositioned. If the system has heaters underneath the workpiece, the lifters may get in the way and also conduct heat.
Another problem with some conveyor systems is that they have stop tabs with downwardly extending positioning pins that pass through holes in the circuit board when in the working position to cause a fine adjustment to the positioning. When the metal plate is lowered after the workpiece is worked on, the board can get caught on one of the pins, causing the system to shut down.
Lifters such as those described above have a useful aspect, however, in that when a lifter is near the center of the board and the plate is raised, the lifter causes the board to bow upwardly. Such a bowed position is generally considered more favorable for dispensing because it requires less vertical movement for the dispenser. It would be desirable to retain this benefit